Plasma application of thermal barrier coatings with reduced thermal conductivity on combustor hardware

ABSTRACT

A process for forming a thermal barrier coating comprises the steps of providing a substrate, providing a gadolinia stabilized zirconia powder, and forming a thermal barrier coating having at least one of a porosity in a range of from 5 to 20% and a dense segmented structure on said substrate by supplying the gadolinia stabilized powder to a spray gun and using an air plasma spray technique.

STATEMENT OF GOVERNMENT INTEREST

The Government of the United States of America may have rights in thepresent invention as a result of Contract No. N00019-02-C-3003 awardedby the Department of the Air Force.

BACKGROUND

The present disclosure is directed to thermal barrier coatings withreduced thermal conductivity on combustor hardware, which coatings areapplied using a plasma.

Ceramic thermal barrier coatings (TBCs) have been used for many years toextend the life of combustors and high turbine stationary and rotatingparts in gas turbine engines. TBCs typically consist of a metallic bondcoat and a ceramic top coat applied to a nickel or cobalt based alloysubstrate which forms the part being coated. The coatings are typicallyapplied to thicknesses between 5 and 40 mils and can provide up to 300degrees F. temperature reduction to the substrate metal. Thistemperature reduction translates into improved part durability, higherturbine operating temperatures, and improved turbine efficiency.Typically, the ceramic layer is a 7 wt % yttria stabilized zirconiaapplied by air plasma spray (APS). New low thermal conductivity coatingshave been developed which can provide improved part performance.

One coating which has been used in the past for TBCs is gadoliniastabilized zirconia based thermal barrier coatings.

SUMMARY OF THE INVENTION

It is desirable to form a thermal barrier coating which has a relativelylow thermal conductivity.

As described herein, there is provided a process for forming a thermalbarrier coating comprises the steps of providing a substrate, providinga gadolinia stabilized zirconia powder, and forming a thermal barriercoating having at least one of a porosity in a range of from 5 to 20%and a dense segmented structure on said substrate by supplying thegadolinia stabilized powder to a spray gun and using an air plasma spraytechnique.

Other details of the thermal barrier coatings applied using an airplasma spray technique, as well as advantages attendant thereto, are setforth in the following detailed description and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are photomicrographs showing a low conductivity crackedcoating formed using a F4 Plasma Spray Gun, which coating includes aceramic layer consisting of 30 wt % Gd₂O₃ and 70 wt % ZrO₂ with a airplasma sprayed MCrAlY bond coat);

FIG. 3 is a photomicrograph showing a coating system which includes ametallic bond coat, a ceramic bond coat, and a ceramic top coat formedby a low conductivity coating in which the metallic bond coat is an airplasma-sprayed MCrAlY bond coat, the ceramic bond coat is a 7 YSZinterlayer, and the ceramic top coat is a 30 wt % Gd₂O₃—70 wt % ZrO₂ toplayer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

As described herein, a plasma spray technique is utilized to apply agadolinia stabilized zirconia based thermal barrier coatings oncombustor hardware such as panels, chambers, heat shields, transitionducts, augmenters, etc. The plasma spray technique may be an air plasmaspray technique in which a desirable coating microstructure is produced.

In air plasma spray, the coating material is propelled toward thesurface of the substrate to be coated. The coating material is in theform of a spray. The powder or powders forming the coating material arefed along with carrier gases into a high temperature plasma gas stream.In the plasma gas stream, the powder particles are melted andaccelerated toward the surface of the substrate to be coated. The powderparticles are fed to a spray gun at a desired feed rate. A carrier gasflow such as an argon gas flow is used to maintain the powder underpressure and facilitate powder feed. The carrier gas flow rate isdescribed in standard cubic feet per hour. Standard conditions may bedefined as about room temperature and about one atmosphere of pressure.

The gases that make up the plasma gas stream comprise a primary gas(such as an argon gas or nitrogen gas) and a secondary gas (such as ahydrogen gas). Helium gas may be used as the secondary gas if desired.

The process includes the step of translating a spray gun so that thenozzle is positioned at a desired distance from the surface to becoated. The substrate to be coated may be passed through the spray ofpowder particles emanating from the spray gun.

The spray gun to be used to form the coatings disclosed herein mayinclude both internal feed and external feed spray guns. Suitable sprayguns include the Plasmadyne SG-100, Sulzer Metco 3 MB, 7 MB, or 9 MB,and the Plasma Technic F-4. The desired coating may also be applied withthe high deposition rate Sulzer Metco Triplex gun and/or the ProgressiveHE100 gun.

Zirconia based powder with the additions of rare earth stabilizers, suchas gadolinia, have been found to yield coatings having lower thermalconductivities than many current thermal barrier coatings. A usefulzirconia based powder is one which consists of optionally 3.0 to 14 wt %of at least one of yttria and titania, 15 to 70 wt % gadolinia, and thebalance zirconia. The yttria and/or titania, and the gadolinia, improvethe thermal barrier coating's ceramic mechanical properties, while stillachieving a reduced thermal conductivity ceramic coating. Coatingsformed using these powders are shown in FIGS. 1 and 2.

If desired, one could apply to a substrate, a coating which has ametallic bond coat, such as a MCrAlY type coating where M is Ni or Co, aceramic interlayer, such as a 7 YSZ coating, deposited on the metallicbond coat and a ceramic top coat comprising from 15 to 70 wt % gadoliniaand the balance zirconia. Such a coating system is illustrated in FIG.3.

The air plasma spray parameters may be adjusted to produce a coatingwith a desired level of porosity or a coating with a dense segmentedstructure. For porous coatings, the coating may have a thermalconductivity which ranges from 3.0 to 10 BTU in/hr ft² F. For segmentedcoatings, the coating may have a thermal conductivity which ranges from5.0 to 12.5 BTU in/hr ft² F.

A useful coating has a porosity in the range of 5.0 to 20%. The desiredporosity for the coating may be obtained by altering the gun powersettings, the standoff distance, the powder particle size, and thepowder feed rate.

Segmented coatings provide the coating with strain tolerance duringoperation which leads to increased spallation life. For combustor panelapplications, a coating system having a segmented microstructure topcoatlayer with a ceramic interlayer provides a useful coating system.

If desired, one can obtain a coating with a dense segmented structure byincreasing the power settings and shorten the standoff distance. One cando this by using the settings set forth in columns 6-8 of U.S. Pat. No.5,879,753, which patent is incorporated by reference herein.

For example, a useful coating may be applied using the Plasmadyne SG-100spray gun using an amperage range of 350 to 825 amps, a voltage of 35 to50 volts applied to a cathode and anode within the plasma-gun body, anargon primary gas flow of 75-105 SCFH, a hydrogen secondary gas flow of1.0 to 10 SCFH or a helium gas flow of 45-75 SCFH, a powder gas flowexiting the gun of 4.0 to 20 SCFH, a powder feed rate to the gun of 10to 40 grams/min., and a gun distance from the surface being coated offrom 3.0 to 5.0 inches. Alternatively, the coatings may be applied withthe Plasma Technic F-4 spray gun using an amperage range of from 500 to700 amps, a voltage of 55 to 65 volts, an argon primary gas flow of 65to 90 SCFH, a hydrogen secondary gas flow of 8-22 SCFH, a powder gasflow from the spray gun of 6 to 12 SCFH, a powder feed rate to the spraygun of 35-55 grams/min. and a gun distance from the surface being coatedof from 4.0 to 7.0 inches.

One of the benefits of the process of the present invention is theapplication of a thermal barrier coating having lower thermalconductivity than many current coatings resulting in longer coatinglife, performance improvements, and cost savings.

Burner rig testing of a low conductivity coating formed in accordancewith the present disclosure with a ceramic interlayer was found to be1.6 to 1.9 times better in spallation resistance than without theinterlayer. In addition, low conductivity coatings with an interlayerare 1.3 to 1.5 times better in spallation than current coatings.

In accordance with the foregoing disclosure, there has been provided aplasma application of thermal barrier coatings with reduced thermalconductivity on combustor hardware. While the plasma application ofthermal barrier coatings has been described in the context of specificembodiments thereof, other unforeseeable alternatives, modifications,and variations may become apparent to those skilled in the art havingread the foregoing description. Accordingly, it is intended to embracethose alternatives, modifications, and variations as fall within thebroad scope of the appended claims.

1. A process for forming a thermal barrier coating comprising the stepsof: providing a substrate; providing a gadolinia stabilized zirconiapowder; and forming a thermal barrier coating having at least one of aporosity in a range of from 5 to 20% and a dense segmented structure onsaid substrate by supplying the gadolinia stabilized powder to a spraygun and using an air plasma spray technique.
 2. The process according toclaim 1, wherein said substrate providing step comprises providing acombustor component.
 3. The process according to claim 1, wherein saidsubstrate providing step comprises providing one of a combustor panel, acombustor chamber, a combustor heat shield, a combustor transition duct,and a combustor augmentor.
 4. The process according to claim 1, whereinsaid powder providing step comprises providing a powder consisting ofoptionally from 3.0 to 14 wt % of at least one of yttria and titania,from 15 to 70 wt % gadolinia, and the balance zirconia.
 5. The processaccording to claim 1, wherein said powder providing step comprisesproviding a powder consisting of from 3.0 to 14 wt % of at least one ofyttria and titanium, from 15 to 70 wt % gadolinia, and the balancezirconia.
 6. The process according to claim 1, wherein said thermalbarrier coating forming step comprises using an amperage range of 350 to825 amps, a voltage of 35 to 50 volts, an argon primary gas flow of 75to 105 SCFH, at least one of a hydrogen secondary gas flow of 1.0 to 10SCFH and a helium secondary gas flow of 45 to 75 SCFH, a powder gas flowexiting a spray gun of 4.0 to 20 SCFH, a powder feed rate to the spraygun of 10 to 40 grams/min., and a gun distance from a surface of thesubstrate being coated of from 3.0 to 5.0 inches.
 7. The processaccording to claim 1, wherein said thermal barrier coating forming stepcomprises using an amperage range of from 500 to 700 amps, a voltage of55 to 65 volts, an argon primary gas flow of 65 to 90 SCFH, a hydrogensecondary gas flow of 8 to 22 SCFH, a powder gas flow from a spray gunof 6 to 12 SCFH, a powder feed rate to the spray gun of 35 to 55grams/min. and a gun distance from a surface of a substrate being coatedof from 4.0 to 7.0 inches.
 8. The process according to claim 1, furthercomprising depositing a ceramic interlayer on said substrate prior tosaid thermal barrier coating step.
 9. The process according to claim 8,wherein said ceramic interlayer depositing step comprises depositing alayer of 7.0 wt % yttria stabilized zirconia.
 10. The process accordingto claim 8, further comprising depositing a bondcoat layer on saidsubstrate prior to said ceramic interlayer depositing step.
 11. Theprocess according to claim 10, wherein said bondcoat layer depositingstep comprises depositing a metallic bondcoat layer.
 12. The processaccording to claim 1, wherein said thermal barrier coating forming stepcomprises forming a segmented coating having a thermal conductivity inthe range of from 5.0 to 12.5 BTU in/hr ft² F.
 13. The process accordingto claim 1, wherein said thermal barrier coating forming step comprisesforming a porous coating having a thermal conductivity in the range offrom 3.0 to 10 BTU in/hr ft² F.